1. Field of the Invention
This invention relates to LED (Light Emitting Diode) based lamps and in particular, although not exclusively, to LED based spotlights.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip generates blue light and the phosphor material(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor material provides light which appears to the human eye as being nearly white in color.
Currently there is a lot of interest in using high brightness white LEDs to replace conventional incandescent light bulbs, halogen reflector lamps and fluorescent lamps. Most lighting devices utilizing high brightness white LEDs comprise arrangements in which a plurality of LEDs replaces the conventional light source component and utilize the existing optical components such as a reflector and/or a lens. Ideally a spotlight would generate an illuminance (luminous flux (power) per unit area incident on a surface) that was substantially uniform across the lamp's emission angle (beam spread). However, as light emission from a lamp is confined within a selected emission angle this can result in a greater proportion of the light emission being concentrated on the axis thereby further reducing illuminance uniformity within the emission angle. Unlike a filament lamp which closely approximates to a point source, LED based lamps generates light which is often far from point source in character requiring the development of new optical arrangements for LED lamps for general lighting applications. A need exists for an LED based spotlight with a selected emission angle of 20° or less.
Co-pending U.S. patent application Ser. No. 12/721,311 filed Mar. 10, 2010 (Publication No. US2010/0237760), to Haitao YANG, teach an LED based downlight comprising a thermally conductive body; a plurality of LEDs configured as an array and mounted in thermal communication with the body; and a light reflective hood located in front of the plane of LEDs. The hood has at least two frustoconical (i.e. a cone whose apex is truncated by a plane that is parallel to the base) light reflective surfaces that surround the array of LEDs and are configured such that in operation light emitted by the lamp is within a selected emission angle. Whilst such a configuration can produce a good uniform illumination for emission angles of 40° and greater such a configuration is unsuitable for spotlights with lower emission angles and in particular spotlights with a compact form factor such as for example MR16 lamps.
U.S. Pat. No. 6,350,041 B1, to Tarsa et al., disclose solid state lamps comprising an elongate separator with a solid state light source (LED) at one end and a disperser at the other end that disperses light in a desired pattern (such as a radially uniform pattern). The disperser can also contain elements, such as phosphors to change the wavelength of at least some of the light from the light source. The separator provides physical separation between the LED and the disperser that is sufficient to prevent substantial heat transfer from the LED to the disperser. In the lamp of FIG. 1 the separator can comprise an optical waveguide and the disperser be hemispherical shaped and can be attached to the end of the separator or formed integrally with the separator. The disperser can include a phosphor for converting the wavelength of at least some of the light from the LED.